Effect of Hf and Hf-Ca substitution on the superconductivity of ErBa2Cu3O7-delta

The structural and superconducting properties of (Er1-x-yCayHfx)Ba2Cu3Oz samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen-content measurements. The effect of increasing the Hf concentration in Er1-xHfxBa2Cu3Oz lowers the oxygen content and decreases T-c which is attributed to hole filling by Hf. The substitution of Ca for Er in (Er0.85-yCayHf0.15)Ba2Cu3Oz provides a proper matching between the ionic radius and valence of Er3+ (0.88 Angstrom) and the average ionic radius and valence of Hf4+ (0.78 Angstrom) and Ca2+ (0.99 Angstrom). As the Ca content increases, the T-c increases from 70 K (y = 0.05) to 86 K (y = 0.25, compensated oxide), closer to the value of 91 K for pure ErBa2Cu3O7-delta due to the balance between the hole filling by Hf and hole doping by Ca.